Offshore oil and gas production in temperate climates whether they be in shallow waters or deep waters can be achieved with production platforms which may be submerged caissons, semi-submersible vessels or tension cable position floatable vessels. However, in climates where icebergs, severe Arctic storms and the like may present hazards to the production vessel, the production platform designs for the temperate climates are not usable. An example of a semi-submersible system which is capable of withstanding icebergs, slab ice, packed ice and severe storm conditions is described in U.S. Pat. No. 5,292,207. The semi-submersible is capable of housing equipment which is ice crush sensitive or unable to withstand severe storms. The system is to some extent duplicite in that in addition to the semi-submersible vessel, drilling platforms and/or production platforms are also required.
Another approach to an offshore platform which resists icebergs is to install a gravity based structure which comprises a concrete monolithic caisson. This structure is intended to remain on the ocean floor and is constructed in a manner to resist icebergs by way of the provision of two concentric peripheral walls designed to withstand iceberg impact. This structure has to be of a substantial construction which is costly and is limited in respect of the depth of waters in which it may be installed.
Canadian Patent 1,209,815 describes a floatable gravity based structure which has an open centre to allow access to the well heads. Once the platform is connected to the foundation the central area is pumped out to allow dry access to the well heads. A negative hydrostatic force is used to restrain the platform on the bottom in preparation of disconnect. Once the equipment is disconnected from the well heads the semi-submersible may be deballasted to float away in the event of danger of collision with icebergs.
U.S. Pat. No. 3,982,401 describes a semi-submersible marine structure which is anchored by a buoyant substructure held in place by tension lines. The connection at the seabed provides for lateral movement of the marine structure due to wave action and wind forces. This structure may be used for either drilling or production, however the system does not readily lend itself to quick connect or disconnect. Deep-sea divers or submarines are required to attach the tension lines to the sea floor anchoring block.
U.S. Pat. No. 4,895,481 describes a floating platform which is connected to the seabed by the use of a flexible structure anchored to the seabed. The upper structure is buoyed by the use of floats which may be metal cylinders. The floats are located sufficiently deep to reduce the hydrodynamic forces induced by swell in the ocean. The floats are adjusted such that the forces exerted on the flexible structure and hence on the conducting tubes is greater than the total compressive force exerted on the conducting tubes of the drilling platform.
Canadian Patent 1,058,978 describes a quick release system for a tension leg offshore production platform. The system has to take away the tension in the tension lines in order to disconnect. There is no provision to ensure that a complete disconnection is made and there is no mention of how the production lines would be disconnected. The system is not designed to handle pack ice or small icebergs and the system does not have a device which would reduce buoyancy fluctuation caused by large waves. In addition hook-up would most likely require deep sea divers to complete the connections of production lines hence this system is useful solely in temperate climates.
Canadian patent 866,577 describes a production platform which is held in place by cables. The system according to one embodiment may include buoyant means on the riser pipe. The buoyant tanks 80 maintain tension in the riser pipe while connection is completed by way of reeling in lines 60 to position and lower the riser pipe into the proper position for connection to the well head. Again deep-sea divers would be required to complete the connection, thus, the system does not lend itself to a quick connect/disconnect.
Canadian patent 1,101,830 describes a disconnectable riser system which allows the drill string to stay in the hole and riser while the drill platform moves out of danger. This type of system is only for drilling in deep water. The riser itself is connected to the platform and is not held in tension. The riser system is buoyed to maintain the riser system in a substantially vertical disposition after being disconnected from the upper riser segment.
Canadian patent 1,204,945 discloses a fixed structure secured to a large subsea foundation. The fixed structure is like a gravity based structure when in operation mode. Stability is obtained by the structure being fixed to the bottom through direct contact with the foundation. The structure is only reusable in waters of the same depth and the foundation is not feasibly reusable. This structure has to be moved for all icebergs and most likely pack ice due to its design. Deep sea divers would be required to disconnect the risers and hence would take several hours to disconnect. At the time of disconnect the structure is held in a state of positive buoyancy where it is firmly seated on the sea bed foundation. The structure is maintained on the sea bed foundation by a hydrostatic force, although mechanical device may also be used. The structure, when connected by mechanical devices to the sub sea foundation, cannot resist any lateral forces because the mechanical connecting devices could be bent and become inoperable. In that event the entire structure would be rendered unmoveable and left in danger in the event of an approaching iceberg or storm. In the event of large sea swells of 15 meters or greater, for example, the structure may oscillate and plunge in a wave valley, striking the foundation on a downward oscillation. In these conditions a disconnect would almost be impossible.
In accordance with an object of an aspect of this invention an offshore oil/gas production platform is provided with a quick connect/disconnect system which involves the use of a submerged connection header. After connection is complete the production vessel is held in a state of positive buoyancy by way of the tension cables holding in turn the submerged connection header in a state of positive buoyancy.